1. Field of the Invention
The present invention relates to amplifiers. More specifically, the invention relates to amplifiers having improved performance characteristics with high-speed switching waveforms, particularly by enhancement of rise times and fall times for high-speed digital signals.
2. Related Art
As appreciated by those skilled in the art, "Class C" defines a class of amplifier which is non-linear, but highly power-efficient. Class C amplifiers are suitable for phase modulation, frequency modulation, or 100% pulse modulation.
Certain Class C amplifiers include an amplifying device, such as a bipolar transistor, as well as "external" circuitry that surrounds the device so that the amplifier as a whole performs its amplifying function.
At the higher frequencies, such as UHF and beyond, the grounded or common base configuration is the preferred configuration, because it offers higher gain than the grounded emitter configuration when using bipolar transistors. A common base amplifier with shunt supply voltage feed is shown in FIG. 1. "RFC" means radio frequency choke, providing a short circuit DC path to supply or ground.
For most applications, the speed at which such an amplifier responds to an RF input is not too important. For example, when an RF input signal is applied to a Class C RF power amplifier stage in a hand-held FM voice communication device, it is sufficient that the amplifier rise to its full output power before modulation is applied. In such applications, rise time may be as long as several hundred microseconds. Also, when the input signal is taken away (such as when reverting to the "receive" mode), fall time as long as several hundred microseconds is acceptable. In another example, FM broadcast transmitters are not concerned with enhancing rise time or fall time, because, as broadcast transmitters, they are turned on or off only once per day or are perpetually on. Similarly, fast rise or fall time is not crucial in pulse modulation using Class C amplifiers in radar applications, with a pulse width as short as one microsecond. If the ON time is considered to be from the 90% ON level to the 90% OFF level, the RF input pulse width may be adjusted to accommodate the slow response of the Class C amplifier.
However, in the modulator disclosed in U.S. Pat. No. 4,804,931 (Hulick), switched Class C amplifiers respond to the commands of an analog-to-digital converter, the purpose of which is to have many amplifiers power combined to synthesize an amplitude modulated output. In that patent, amplifiers must respond to the ON/OFF commands at a speed fast enough to avoid significant switching distortion. If a digitized video signal derived from an analog television video signal having a 4.2 MHz bandwidth is sampled at a frequency at least twice this bandwidth, each Class C amplifier may have to switch from OFF to ON to OFF in a total time span of no more than 119 nanoseconds. Clearly, Class C amplifiers requiring several microseconds or even several hundred nanoseconds to switch, cannot fill this need. Linear amplifiers can be used in modulators such as that in U.S. Pat. No. 4,804,931, but linear amplifiers are less power efficient and are therefore not a viable solution.
FIG. 2 is an oscilloscope trace of the detected RF output of a typical high power Class C UHF transistor amplifier driven with an input RF pulse having 10 ns rise and fall times. The circuit causing the illustrated output is a specific design derived from FIG. 1, using a 60 watt device operating at 645 MHz. In the illustrated example, rise time deteriorates to 300 ns and fall time deteriorates to 125 ns. This deterioration is unacceptable in many applications, for reasons outlined above.
FIG. 3 shows idealized waveforms for graphically demonstrating the rise time/fall time deterioration phenomena, as a background for understanding the motivation behind edge enhancement. Waveform (a), an idealized RF input pulse, is assumed to be input to a typical Class C amplifier, whose response is shown as output Waveform (b); rise time deterioration is t.sub.2 -t.sub.1, and fall time deterioration is t.sub.4 -t.sub.3.
The FIG. 1 conventional circuit's edge deterioration is believed to derive from the following causes. The transistor fails to turn on quickly because "on" bias must be derived from the input signal; until an input is applied, the transistor is biased off, causing deterioration in rise time. Conversely, fall time suffers because stored charge in the device must bleed on its own, because no help is provided with external circuitry.
Various amplifiers, and modifications and enhancements to amplifiers, are known in the art. U.S. Pat. No. 4,644,293 (Kennett) discloses an RF pulse modulated amplifier which transitions from Class A through Class AB to Class C operation. External circuitry is provided to sense the input signal and provide a control current which varies in accordance therewith. U.S. Pat. No. 4,924,191 (Erb et al.) discloses an amplifier having digital control over the operating point of several transistors. Digital control circuitry is used to sense the input signal and the operating environment, and provide biasing to the transistors. U.S. Pat. No. 4,460,876 (Najman) discloses an amplifier which operates in a "sliding Class A" mode by varying the bias on the output transistor as a function of the amplitude of the input signal. Finally, U.S. Pat. No. 4,087,761 (Fukumoto et al.) discloses an amplifier which can be operated in Class A, B, or AB by selectively switching bias values. The amplifier continues to operate in a selected mode continuously until manually switched. These patents, as well as all documents cited in this specification, are incorporated herein by reference as if reproduced in full.
Unfortunately, none of the known systems provides a way of improving rise times and fall times which automatically reacts to both rise time periods and fall time periods, so as to provide a self-adjusting compensation which improves rise times and fall times. It is to fill this need that the present invention is directed.